1. Field of the Invention
The invention is generally related to pipelines and more particularly to an internally insulated, corrosion resistant configuration.
2. General Background
In the offshore production of oil and gas, subsea flowlines transport fluids produced from the offshore gas and/or oil wells to a production station located some distance from the well. The contents of a flowline typically include oil, natural gas, and water. Subsea pipelines transport a single fluid such as oil, gas, or water for the purposes of export or injection. The mixture is at a warm temperature when first leaving the reservoir, but the contents cool as they flow through the line, with the cooling rate being rapid when the production flow is stopped (shut-in). When the flow line contents cool to sufficiently low temperatures, two serious problems can occur, hydrate formation and wax deposition. Hydrates form at sufficiently low temperatures when natural gas and water combine into an ice-like structure. Wax deposition begins when the walls of the pipe cool below the cloud point of the oil.
Insulating sub sea flow lines has been used as a means of minimizing steady-state heat loss from a flowing pipeline as well as to prolong the cool down of the contents once flow has been stopped. There are several commercially available insulation materials for application to sub sea flow line transport of fluids produced from offshore gas/oil wells. These include non-jacketed and pipe-in-pipe insulation. A non-jacketed insulation is coated directly on the exterior of a pipe. Pipe-in-pipe configurations include an insulation medium in the annulus between the inner pipe (carrier) and the outer pipe (jacket). Conventional external insulation is limited by pressure carrying ability and water absorption at deeper water depths. Conventional pipe-in-pipe technology uses two steel pipes fabricated together to form an annulus that is insulated by some means which may include polyurethane foam (PUF), insulating micro-spheres, or a vacuum. The resulting product is highly effective but costly to manufacture and install.
Methods other than insulation for keeping the pipe contents warm include active heating of the flow line by electrical or other means. In addition, injection of chemical inhibitors has been used to prevent wax deposition and hydrates. These methods are costly as well.
Another problem in many production applications is corrosion. The presence of H.sub.2 S (hydrogen sulfide) and CO.sub.2 (carbon dioxide) in a production stream or the transport of any corrosive fluid will cause serious corrosion of carbon steel pipes. Current methods to deal with corrosion include using corrosion resistant alloys (CRA's), either as a solid pipe or as a cladding on lining of a carbon steel pipe, corrosion resistant coatings on the inside of carbon steel, and use of a thermoplastic liner inside of carbon steel.
The shortcoming of CRA's and coatings is they are usually expensive. The shortcoming of thermoplastic lined pipe is that the permeability of the thermoplastic liner allows gas to pass through the liner and build up in the inter-annular region between the liner and the pipe. This mechanism can lead to liner collapse and/or failure of the liner system.
The known art leaves a need for improved configurations.